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Probiotics are "live microbial dietary supplements which beneficially affect the host animal by improving its intestinal microbial balance. Antibiotics and other related compounds are not included in this definition. In humans, lactobacilli are commonly used as probiotics, either as single species or in mixed culture with other bacteria. Other genera that have been used are bifidobacteria and streptococci." Lactobacilli have been used in the food industry for many years, because they are able to convert sugars (including lactose) and other carbohydrates into lactic acid. This not only provides the characteristic sour taste of fermented dairy foods such as yogurt, but acts as a preservative, by lowering the pH and creating fewer opportunities for spoilage organisms to grow.
Proposed mechanism of action
The rationale for probiotics is that they restore the normal microbes in the gastrointestinal tract that may have been reduced by use of antibiotics or other conditions.
There is no published evidence that probiotic supplements are able to replace the body’s natural flora when these have been killed off; indeed bacterial levels in feces disappear within days when supplementation ceases. It is hoped, however, that probiotics do form beneficial temporary colonies which may assist the body in the same functions as the natural flora, while allowing the natural flora time to recover from depletion. The probiotic strains are then thought to be progressively replaced by a naturally developed gut flora. If the conditions which originally caused damage to the natural gut flora persist, the benefits obtained from probiotic supplements will be short lived.
Experiments into the benefits of probiotic therapies suggest a range of potentially beneficial medicinal uses for probiotics. For many of the potential benefits, research is limited and only preliminary results are available.
Managing Lactose Intolerance
Prevention of Colon Cancer
In laboratory investigations, LAB have demonstrated anti-mutagenic effects thought to be due to their ability to bind with heterocyclic amines; carcinogenic substances formed in cooked meat. Animal studies have demonstrated that LAB can protect against colon cancer in rodents, though human data is limited and conflicting. Most human trials have found that LAB may exert anti-carcinogenic effects by decreasing the activity of an enzyme called ß-glucuronidase (which can generate carcinogens in the digestive system). Lower rates of colon cancer among higher consumers of fermented dairy products have been observed in some population studies.
Animal studies have demonstrated the efficacy of a range of lactobacilli to be able to lower serum cholesterol levels, presumably by breaking down bile in the gut, thus inhibiting its reabsorption (which enters the blood as cholesterol).Some, but not all human trials have shown that dairy foods fermented with LAB can produce modest reductions in total and LDL cholesterol levels in those with normal levels to begin with, however trials in hyperlipidemic subjects are needed.
Lowering Blood Pressure
everal small clinical trials have shown that consumption of milk fermented with various strains of LAB can result in modest reductions in blood pressure. It is thought that this is due to the ACE inhibitor-like peptides produced during fermentation.
Improving Immune Function and Preventing Infections
Lactobacilli are thought to have several presumably beneficial effects on immune function. They may protect against pathogens by means of competitive inhibition (i.e., by competing for growth) and there is evidence to suggest that they may improve immune function by increasing the number of IgA-producing plasma cells, increasing or improving phagocytosis as well as increasing the proportion of T lymphocytes and Natural Killer cells. Clinical trials have demonstrated that probiotics may decrease the incidence of respiratory tract infections and dental caries in children LAB foods and supplements have been shown to be effective in the treatment and prevention of acute diarrhea; decreasing the severity and duration of rotavirus infections in children as well as travelers diarrhea in adults. A diet including milk fermented with Lactobacillus bacteria prevented Salmonella infection in pigs. Claims are made that probiotics strengthen the immune system. 
A meta-analysis suggested probiotics may reduce antibiotic-associated diarrhea. A subsequent randomized controlled trial showed that consumption of a probiotic drink containing L casei, L bulgaricus, and S thermophilus can reduce the incidence of antibiotic associated diarrhoea and Clostridium difficile associated diarrhea in elderly patients.
Upper respiratory tract infection
"Probiotics were better than placebo in reducing the number of participants experiencing episodes of acute URTIs, the rate ratio of episodes of acute URTI and reducing antibiotic use" according to the Cochrane Collaboration. 
LAB foods and supplements have been found to modulate inflammatory and hypersensitivity responses, an observation thought to be at least in part due to the regulation of cytokine function. Clinical studies suggest that they can prevent reoccurrences of inflammatory bowel disease in adults, as well as improve milk allergies and decrease the risk of atopic eczema in children.
Improving Mineral Absorption
It is hypothesized that probiotic lactobacilli may help correct malabsorption of trace minerals, found particularly in those with diets high in phytate content from whole grains, nuts, and legumes.
Prevents Harmful Bacterial Growth Under Stress
In a study done to see the effects of stress on intestinal flora, rats that were fed probiotics had little occurrence of harmful bacteria latched onto their intestines compared to rats that were fed sterile water.
Irritable Bowel Syndrome and Colitis
B. infantis 35624, sold as Align, was found to improve some symptoms of irritable bowel syndrome (IBS) in women. Another probiotic bacterium, Lactobacillus plantarum 299V, was also found to be effective in reducing IBS symptoms.  Additionally, a probiotic formulation, VSL3, was found to be effective in treating ulcerative colitis  Bifidobacterium animalis DN-173 010 may help. 
Prebiotics are "nondigestible food ingredients that beneficially affect the host by selectively stimulating the growth and/or activity of one or a limited number of bacterial species already resident in the colon, and thus attempt to improve host health".
Synbiotics have also been defined as the synergistic action of prebiotics and probiotics. As probiotics are proposed to be active in the small intestine and prebiotics are proposed be to be effective in the large intestine, the combination of the two may give a synergistic effect. Appropriate combinations of pre- and probiotics are synbiotics.
The most common form for probiotics are dairy products and probiotic fortified foods. However, tablets and capsules containing the bacteria in freeze dried form are also available.
There is currently only one species of yeast used as a probiotic:
Some commonly used bacteria in products, but without probiotic effect (yogurt bacteria):
Some other bacteria mentioned in probiotic products:
- Bacillus coagulans
- Lactobacillus bifidus - became new genus Bifidobacterium 
- Lactobacillus caucasicus, a fantasy name, as no species with this name exists.
Some fermented products containing similar (often not proven to have a probiotic or health effect) lactic acid bacteria include:
Some researchers are skeptical of some of the claims made for probiotics. Their reasons include the following considerations:
- The studies done in support of probiotics are mostly anecdotal or heavily reliant on test-tube experimentation rather than on clinical trials in human subjects
- The basic concept of probiotics is based on a misunderstanding of the role of microflora in the human digestive tract.
- It is difficult to see how bacteria taken by mouth can survive the process of human digestion (though research shows that they do, in fact, survive )
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